1. Field of the Invention
The present invention relates to an automobile interior finish member having a door suitable for use with an air bag device. Examples of such an interior finish member are an instrument panel, a door trim and a seat back into which an air bag device is installed. More particularly, the present invention relates to a composition and a manufacturing method of the interior finish member wherein the interior finish member provides two necessary yet incompatible properties of rigidity and flexibility.
2. Description of the Related Art
A conventional instrument panel of this type of interior finish member comprises a main body portion and a cover portion for an air bag, as disclosed by Japanese Patent Publication No. Hei. 5-162603. The main body portion of the instrument panel and the cover portion are made of rigid thermoplastic resin. The main body portion of the instrument panel and the cover portion are integrally formed in such a manner that surfaces of the main body portion and the cover portion form a continuous surface. The cover portion includes: a door portion which deploys under a force of an expanding air bag, providing an opening through which the expanding air bag can pass; a hinge portion constructed and arranged to provide a pivot around which the door portion can open; and a breakable portion, defined by a notched periphery of the cover portion wherein the wall thickness of the periphery is thinner than adjacent portions of the instrument panel.
An instrument panel must be both rigid and flexible. The main body portion of the instrument panel must be weather and heat resistant as well as rigid enough to resist impact and preserve its design shape. On the other hand, the cover portion of the instrument panel is required to have a breakable portion which does not fragment in its breaking process. Accordingly, a hinge portion of the cover portion, which functions as a fulcrum during deployment of the door portion, must be flexible yet have sufficient tensile strength to resist separating from the main body (hereinafter the "deployment property").
However, since rigidity is incompatible with flexibility, it is difficult to select a material satisfying both properties. As a result, the wall thickness of the hinge portion and the wall thickness of the breakable portion around a periphery of the door portion must be carefully controlled to accommodate the two requisite properties.
To provide the rigidity necessary to resist impact and preserve the design shape, both the main body and the cover portion of the conventional instrument panel are made of fiber reinforced thermoplastic resin (FRTP) which provides the necessary rigidity and impact resistance. Such fiber reinforced thermoplastic resin provides stiffness in the portion to be severed. To provide the flexibility necessary in the hinge portion, the wall thickness in the hinge portion is reduced. However, when the wall thickness is reduced, it becomes difficult to provide the necessary tensile strength of the hinge portion.
Therefore, providing the requisite rigidity, flexibility and tensile strength of the hinge portion becomes a formidable task. In the conventional interior finish member, the difficulties in selecting a material and carefully controlling the wall thickness is handled by reducing the wall thickness of the hinge portion and disposing a strong but flexible reinforcing member, such as a net made of aramid fiber, in the hinge portion.
However, when such a reinforcing member is used during the manufacturing of the conventional instrument panel, it is necessary for a worker to manually set the flexible reinforcing member at a predetermined position in the mold. This process expends time and labor costs.
To illustrate the problem associated with this type of conventional interior finish member, an instrument panel 310 having a substantially curved section is shown in FIG. 22. The instrument panel 310 comprises: a main body 311; a cover portion 312; and a square tube-shaped attaching wall (shown partially by a front attaching wall 324a and a rear attaching wall 324b) disposed on a back side of the cover portion 312 and connected to an air bag device. In this connection, a facing layer 117 manufactured by means of insertion formation is provided for the purpose of enhancing the design.
Manufacture of the instrument panel 310 is conducted by a stationary mold 340 and a movable mold 338 moved in a direction perpendicular to the maximum expanding portion on a curved surface (shown in the direction of the arrow in FIG. 22). This process prevents the occurrence of an under-cut in both the stationary mold 340 and the movable mold 338. However, the front attaching wall 324a and the rear attaching wall 324b, which are formed substantially perpendicular to an upper surface of the instrument panel 310, cross a mold releasing direction, so that the undercut is formed.
Moreover, a first sliding core 126 and a second sliding core 128 are necessary in the mold releasing directions of the front attaching wall 324a and the rear attaching wall 324b. As a result, the structure of the metallic mold becomes complicated.